x86/pmu: Track GP counter and event select base MSRs in pmu_caps

Snapshot the base MSRs for GP counters and event selects during pmu_init()
so that tests don't need to manually compute the bases.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Like Xu <likexu@tencent.com>
[sean: rename helpers to look more like macros, drop wrmsr wrappers]
Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221102225110.3023543-19-seanjc@google.com